The invention relates to a display tube comprising in an evacuated envelope a display screen of a luminescent material provided on the innerside of a display window in the wall of the envelope, and a multilayer interference filter disposed between said material and the display window, said filter comprising a plurality of layers manufactured alternately from a material having a high refractive index and a material having a low refractive index.
Display tubes have a wide field of application. They are used, for example, as projection television display tubes, oscilloscope tubes, X-ray amplifier tubes, D.G.D.-tubes (DGD=Data Graphic Display), or as television display tubes.
Such a display tube is known from German Patent Application No. 2330898 laid open to public inspection, in which a multilayer interference filter is used between the luminescent material (the phosphor) and the display window. In the case of layers of the filter having a material with a high refractive index the optical thickness of each layer is chosen to be smaller than 0.25.lambda. or between 0.5 and 0.75.lambda. wherein .lambda. is the wavelength of the light emitted by the luminescent material. For the layers of the fiter having a material with a low refractive index an optical thickness of 0.25.lambda. is used or an odd multiple thereof. The light reflection characteristic of the display screen of the tube is varied by the filter so that the quantity of light reflected within a limited angular aperture increases by 25% as compared with a tube without a filter. At the same time the contrast in the produced picture is increased by a reduction of the background brightness.
A similar tube is disclosed in British Patent Specification No. 1,306,335 in which a so-called pass-band interference filter is provided between the phosphor and the display screen. This filter consists of layers having an optical thickness of 0.25.lambda. and has the following composition: S L H L L L L L L H L H L L L L L L H L H L L L L L L H, wherein S is the dislay screen, L are layers of a material having a low refractive index and H are layers from a material having a high refractive index. Such a filter may also be written as follows: EQU S L H (L).sup.6 H L H (L).sup.6 H L H (L).sup.6 H.
As a result of the presence of the six layers placed against each other and having a low refractive index (L).sup.6, a Fabry-Perot filter is formed. With a given choice of the optical layer thickness, the filter has a desired transmission band for light rays which maker an angle smaller than 25.degree. to 40.degree. to the normal on the filter. In addition, however, there is an undesired wide transmission band for light rays which enclose an angle between 55.degree. and 90.degree. to the normal. All the light which is passed in this band is lost or contributes to the so-called halo and as a result of this to loss of contrast in the displayed picture. This will be explained in detail hereinafter. The cryolite layers used in this filter moreover are hygroscopic and remain soft, so that damage to the filter can easily occur. The zinc sulphide layers (ZnS) used in this filter easily oxidize when the tube is fired at approximately 460.degree. C., so that they are less suitable for use in display tubes.
The article "Anti-halo coatings for cathode ray tube face plates", by J. D. Rancourt, Proceedings of the SID, vol.25/1 discloses an 11-layer "edge" filter and an 8-layer "pass-band" interference filter for suppressing halo effects. It is also suggested to use one of these filters for projection-T.V. (pag. 47, top right). An 11-layer filter, however, for projection-T.V. gives much loss because too much light is passed at large angles.